1. Field of the Invention
This invention generally relates to light waveguide mediums and, more particularly, to a system and method for controlling the collimation of light extracted from a backlight device.
2. Description of the Related Art
FIG. 1 is a plan view of representing light extracted from a liquid crystal display (LCD) backlight (prior art). Mura is a Japanese term for unevenness, inconsistency in physical matter, or human spiritual condition. This word is used in LCD to describe undesired illumination non-uniformity due to design or fabrication defects. Mura can come from both front and back panels. As shown in the figure, more light is being extracted near the input light emitting devices (LEDs) on the left side of the panel, than on the right side of the panel. The significant amount of light extracted near the light source leaves an insufficient amount of light to be extracted from the right side of the panel. Backlight panels are conventionally designed using a significant degree of trial-and-error to find the correct balance of light extraction and illumination.
FIG. 2 is a partial cross-sectional view of a liquid crystal display (LCD) backlight system (prior art). Ideally, the system is intended to extract and collimate light (from the light source) up, through the waveguide top surface, to illuminate an LC panel (not shown).
FIG. 3 is a diagram comparing the intensity of light extracted from the waveguide top and bottom surfaces (prior art). Often, as shown in FIG. 2, a reflection pattern is added to the bottom of the waveguide to minimize the amount of light exiting the waveguide through the bottom surface. Alternatively or in addition, as shown in FIG. 2, a reflector can be added under the waveguide bottom surface. However, both these solutions undesirably increase the thickness and complexity of the backlight system.
FIG. 4 is a partial cross-sectional view of a waveguide with a light extraction feature (prior art). With an LED 400 light input of 0° (parallel to the waveguide top surface 404), the maximum angle of light propagation α′ inside the waveguide is:Ω˜sin−1(1/nW)
where nW is the index of waveguides. For a conventional backlight polymer with a refractive index ˜1.49, the angle Ω is roughly 42°. Light rays within the cone of ±42° (where horizontal is 0°) result is light rays exiting the waveguide top surface in a cone of ±48° (where vertical is 0°). However, in different situations, either greater or lesser amounts of light collimation are desired.
With the addition of air bubble light extraction features 406 and a design that avoids the critical angle for total internal reflection, the light rays within the horizontal cone may be directed toward the top surface for light extraction with a greater degree of dispersion. In the best case dispersion scenario, light inside this angular cone of ±42° (horizontal) can be extracted within ˜±32° (vertical). Increasing the dispersion of extracted light is useful for wide angle viewing.
FIG. 5 is a partial cross-sectional view of a waveguide light extraction feature with various angles of incident light (prior art). Depending on where light strikes the bubble structure, it may be reflected to the waveguide bottom surface (the rays marked “1”), extracted from the waveguide top surface due to total internal reflection (the rays marked “2”), or realigned at an angle where it is likely to strike another bubble structure at a favorable angle (the rays marked “3”). Conventionally, rays (1) reflected to the waveguide bottom surface have been an undesirable limitation associated with the use of light extraction features.
As noted in the application entitled, ULTRA-THIN WAVEGUIDE WITH CONTROLLED LIGHT EXTRACTION, invented by Jiandong Huang et al., Ser. No. 13/484,346, filed May 31, 2012, bubble structures can be used to enhance the degree of light collimation, as might be useful in narrow (private) angle viewing.
It would be advantageous if backlight panels and waveguide devices could be more efficiently designed to control the collimation of extracted light from a specified waveguide surface.